1. Field of the Invention
The present invention relates to high density electrically-erasable, programmable read-only-memory (EEPROM) devices and, in particular, to a method of fabricating a high-density EEPROM cell having reduced read time.
2. Discussion of the Related Art
The basic, fundamental challenge in creating an electrically-erasable programmable read only memory (EEPROM) cell is to use a controllable and reproducible electrical effect which has enough nonlinearity so that the memory cell can be written or erased at one voltage in less than 1 ms and can be read at another voltage, without any change in the programmed data for more than 10 years. Fowler-Nordheim tunneling exhibits the required nonlinearity and has been widely used in the operation of EEPROM memories.
In silicon (Si), the energy difference between the conduction band and the valence band is 1.1 eV. In silicon dioxide (SiO.sub.2), the energy difference between these bands is about 8.1 Ev, with the conduction band in SiO.sub.2 3.2 Ev above that in Si. Since electron energy is about 0.025 Ev at thermal room temperature, the probability that an electron in Si can gain enough thermal energy to surmount the Si-to-SiO.sub.2 barrier and enter the conduction band in SiO.sub.2 is very small. If electrons are placed on a polysilicon floating gate surrounded by SiO.sub.2, then this band diagram will by itself insure the retention of data.
Fowler-Nordheim emission, which was observed early in this century for the case of electron emission from metals into vacuums, was also observed by Lenzliger and Snow in 1969 for electron emission from silicon to silicon dioxide. In the presence of a high electric field at the Si-SiO.sub.2 interface, the energy bands will be distorted and there is a small, but finite, probability that an electron in the conduction band of the Si will quantum mechanically tunnel through the energy barrier and emerge in the conduction band of the SiO.sub.2.
The tunneling current increases exponentially with the applied field in accordance with the following general current density expression: EQU J=(AE2)exp(-BIE)
where A and B are constants, and PA1 E is the field at the Si-SiO.sub.2 interface
This current is observable at a current density of 10 E-6 A/cm2 when the field at the Si-SiO.sub.2 interface is about 10 MV/cm. Local fields of this magnitude, at voltages practicable for use in microelectronics, can be obtained by applying a voltage across either a thin (about 100 .ANG.) oxide grown on bulk silicon or across thicker (about 500 .ANG.) oxide grown on polysilicon. In the latter case, the field enhancement arises from textured polysilicon formation, i.e. positive curvature regions at the polysilicon-polysilicon oxide interface resulting in tunneling enhancement at similar voltages as in the first case.
The theoretically ideal EEPROM memory cell comprises a single transistor addressable by applying electrical signals to a specified row and a specified column of the memory array matrix. For example, to write a logic "1" or a logic "0" into this "ideal" cell, a voltage is applied to the control gate corresponding to the row (word line) of the selected cell while a voltage corresponding to either a "1" or a "0" is applied to the source or drain corresponding to the column (bit line) of the selected cell.
An important problem encountered in attempts to realize this "ideal" cell is the need for an additional access transistor in each memory cell to enable selection of a single row of memory cells while changing data in the selected cell without accidentally writing or erasing data stored in other rows. Unfortunately, the presence of an additional access transistor in each memory cell increases the size of the cell and leads to impractical die size for high density, megabit memory arrays.
It is, therefore, a goal to provide an EEPROM cell which does not require an additional distinct access transistor in each memory cell to provide reliable selection of a single cell for changing data while precluding accidental simultaneous programming or erasure in non-selected cells.
FIG. 1A shows a cross-section of the well-known FLOTOX EEPROM memory cell. In the FLOTOX cell, the tunnel oxide, which typically is less than 100 .ANG.thick, is grown over an area defined photolithographically in the drain region (or an extension of the drain region, called buried n+). Charging of the floating gate to program the cell is achieved by grounding the source and the drain and applying a high voltage to the control gate. The FLOTOX cell is designed such that a large fraction of the applied voltage is coupled across the tunnel oxide, resulting in the transport of electrons from the drain to floating gate. Discharge of the floating gate to erase the cell is achieved by grounding the control gate, floating the source and applying a high voltage to the drain. In this case, most of the applied voltage is coupled across the tunnel oxide, but the field is reversed, resulting in tunneling of electrons from the floating gate to the drain. The source is floated so that there is no continuous current path, an important factor when an internal charge pump is used to generate the high voltage from a .ltoreq.5 V supply.
If a single transistor memory cell is placed in a typical array with drains connected to metal columns and gates connected to common polysilicon word lines, the erasing of the cell, with the word line grounded, will mean that high voltage is applied to all drains in a common column. Erasing can be inhibited in non-selected cells by taking unselected word lines to a high voltage. However, this means that unselected cells along the same word line may be programmed. To avoid such disturb conditions, as shown in FIG. 1A, the FLOTOX cell utilizes a distinct access transistor to isolate the drain from the column bit line. The access transistor is off for rows that are not selected for erasure.
FIG. 1B shows a layout of the FIG. 1A FLOTOX cell, with the FIG. 1A cross-section being taken perpendicular to the word line (control gate) and through the tunnel oxide window.
FIGS. 2A-2G illustrate a process flow sequence utilized for fabricating the FIG. 1A FLOTOX cell. As shown in FIG. 2A, the fabrication sequence begins with the formation of an oxide layer 10 on a silicon substrate 12, followed by the patterning of a photoresist mask 14 and an ion implant step to form the buried n+ regions 16 of the EEPROM memory cell.
As shown in FIG. 2B, following the formation of the buried n+ regions 16, a tunnel window opening 18 is etched in the oxide layer 10 utilizing a second photoresist mask 20. A thin layer of tunnel oxide 22, approximately 80 .ANG.thick, is then grown in the tunnel window, as shown in FIG. 2C.
Referring to FIG. 2D, following growth of the tunnel oxide 22, a first layer of polysilicon is deposited and doped to a desired conductivity. This is followed by formation of an oxide/nitride/oxide (ONO) layer over the first polysilicon layer. The ONO and underlying first polysilicon layer are then masked and etched to define the polysilicon floating gate 24 of the memory cell with an overlying ONO layer 26. Reoxidation and etchback results in the formation of oxide sidewall spacers 28 on the edges of the floating gate 24 and ONO 26.
Referring to FIG. 2E, a second layer of polysilicon is then deposited and doped to a desired concentration and then etched to define a control gate 30 of the memory cell and the gate 32 of the access transistor of the FLOTOX cell. An N+ source/drain/implant is then performed to further define the memory cell and the source/drain regions 34 of the access transistor, as shown in FIG. 2F.
Finally, a layer of dielectric material 36 is formed and planarized and then etched to form a contact opening to the N+ drain/bit line 34. This is followed by formation of a metal bit line structure 38, resulting in the FLOTOX cell shown in FIG. 2G (which is identical with the FIG. 1A cell).
The FLOTOX cell suffers from a number of disadvantages. First, it is susceptible to misalignment between the tunnel window and the buried N+ region of the memory cell. The second layer of polysilicon is used to form the word line of the memory cell and the access transistor gate. However, there is no poly 1/poly 2 self-aligned etch to allow definition of the poly 1 and poly 2 gates of the memory cell transistor. Furthermore, the cell is susceptible to misalignment between the poly 2 access transistor gate and the poly 1 floating gate of the memory cell.
E. K. Shelton, "Low-power EE-PROM can be reprogrammed fast", Electronics, Jul. 31, 1980, pp 89-92, discloses a basic EEPROM concept similar to the above-described FLOTOX concept. However, as shown in FIG. 3, instead of a tunnel oxide area defined lithographically over the drain (buried N+), the Shelton cell has its tunnelling area defined in the channel under the polysilicon floating gate. The polysilicon floating gate partially spans the drain side of the channel, while the remainder of the channel (source side) is spanned by an overlying aluminum control gate. The aluminum control gate is insulated from the polysilicon floating gate by a thin silicon nitride layer.
Furthermore, the Shelton memory cell is formed in a P-well on a N-substrate. Controlling the P-well potential allows the elimination of the distinct access transistor in each memory cell. The potential of the P-well and the sources and drains of the unselected cells are chosen during programming operations to prevent minority carriers from discharging any of the floating gates to the substrate while permitting an individual selected floating gate to be programmed.
Programming of the FIG. 3 cell is achieved by grounding the P-well and connecting the drain through a load resistance to the programming voltage. The AT source is connected to either the programming voltage or to ground depending upon whether a "1" or a "0" is to be stored. To initiate programming, the aluminum control gate is connected to the high voltage. If the source potential is also connected to the high voltage, then the internal access transistor doesn't turn on and the surface of the P-well below the floating gate is depleted of electrons. Only a small potential difference exists between the surface of the P-well and the floating gate. Therefore, no electrons tunnel into the gate and the cell remains in a 0 state. If the source terminal is connected to ground (to program a 1), then the internal access transistor turns on, the surface potential under the floating gate drops to close to 0 V, and electrons from the inversion layer tunnel through the thin oxide into the floating gate.
The FIG. 3 cell is erased by grounding the control gate and then raising the P-well to the programming voltage. This causes electrons to tunnel from the floating gate to the P-well via the tunnel oxide. As electrons tunnel through the tunnel oxide, the floating gate acquires a net positive charge.
Although the FIG. 3 Shelton cell differs from the FIG. 1 FLOTOX cell in that it does not utilize a distinct access transistor, it does require an internal access transistor and, thus, also requires a relatively large cell size.
Commonly-assigned U.S. Pat. No. 5,379,253, issued Jan. 3, 1995, to Albert Bergemont, discloses a memory cell wherein neither a distinct access transistor (as in the FLOTOX cell) nor an internal access transistor (as in the Shelton cell) is required to isolate a memory cell, which has been selected to be programmed, from an adjacent memory cell, which has not been selected to be programmed. As a result, the die size of a high-density EEPROM array constructed from the memory cells described in the '253 patent is smaller than the die size of an EEPROM array constructed from either the FLOTOX cell or the Shelton cell.
As is well known, the time required to read a memory cell can be a significant factor in the selection of an EEPROM device. Although the memory cell described in the '253 patent eliminates the need for a distinct access transistor and an internal access transistor, the time required to read a cell, regardless of the cell type utilized to construct the array, remains substantially the same. Thus, there is a need for an EEPROM array that significantly reduces the time required to read a cell of the array.